Communication device and communication method

ABSTRACT

A communication device is provided with a turbo encoder ( 1 ) which carries out a turbo encoding process on the lower two bits of transmission data so as to output two-bit information bits and two-bit redundant bits that are generated so as to uniform the correction capabilities with respect to the respective information bits; a turbo decoder (a first decoder ( 11 ), a second decoder ( 15 ), etc.) which carries out a soft-judgment on the lower two bits of a received signal that are susceptible to degradation in characteristics, and also carries out an error-correction process using Reed Solomon codes thereon so that the information bits of the lower two bits are estimated, and a third judging device ( 22 ) which carries out a hard-judging process on the other bits of the received signal so that the other upper bits are estimated.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP00/08826 which has an Internationalfiling date of Dec. 13, 2000, which designated the United States ofAmerica and was not published in English.

TECHNICAL FIELD

The present invention relates a method of and device for communicationsthat use a multi-carrier modem system, and, more particularly, concernsa communication device which realizes data communication through theexisting communication lines by using a system such as the DMT (DiscreteMulti Tone) modem system and the OFDM (Orthogonal Frequency DivisionMultiplex) modem system and a communication method for such acommunication device. However, the present invention is not intended tobe limited to the communication device for carrying out datacommunication through the DMT modem system, and is applicable to anycommunication device for carrying out cable communication and radiocommunication through normal communication lines by using themulti-carrier modem system and a single carrier modem system.

BACKGROUND ART

The conventional communication methods will be explained here. Forexample, in the wide band CDMA (W-CDMA: Code Division Multiple Access)using the SS (Spread Spectrum) system, turbo codes have been proposed aserror-correction codes that greatly exceed convolutional codes in theirperformances. In the turbo code, a list formed by interleaving aninformation list is encoded in parallel with a known coding list, andthe turbo code is one of the error-correction codes that have attractedthe greatest public attention at present, and is said to providecharacteristics close to Shannon limit. In the above-mentioned W-CDMA,since the performances of the error-correction code give great effectson the transmission characteristics in the voice transmission and datatransmission, the application of the turbo code makes it possible togreatly improve the transmission characteristics.

Operation of transmitting and receiving systems of a conventionalcommunication device using the turbo code will be explained in detail.FIG. 6 is a drawing that shows the construction of a turbo encoder usedin the transmitting system. In FIG. 6(a), reference number 101 is afirst recursive system convolutional encoder that subjects aninformation list to a convolutional encoding process to output redundantbits, 102 is an interleaver, and 103 is a second recursive systemconvolutional encoder that subjects the information list that has beenswitched by the interleaver 102 to a convolutional encoding process tooutput redundant bits. FIG. 6(b) is a drawing that shows the innerstructures of the first recursive system convolutional encoder 101 andthe second recursive system convolutional encoder 103, and the tworecursive system convolutional encoders are encoders that only outputredundant bits respectively. Moreover, the interleaver 102, which isused in the turbo encoder, randomly switches information bit lists.

The turbo encoder, which is arranged as described above, simultaneouslyoutputs an information bit list: x₁, a redundant bit list: x₂ obtainedby encoding the information bit list through the operation of the firstrecursive system convolutional encoder 101, and a redundant bit list: x₃obtained by encoding the information bit list that has been interleavedthrough the operation of the second recursive system convolutionalencoder 103.

FIG. 7 is a drawing that shows the construction of the turbo decoderthat is used in the receiving system. In FIG. 7, reference number 111indicates a first decoder that calculates a logarithm likelihood ratiofrom the received signals y₁ and y₂. Reference numbers 112 and 116indicate adders, 113 and 114 indicate interleavers, 115 indicates asecond decoder that calculates a logarithm likelihood ratio from thereceived signals y₁ and y₃. Reference number 117 indicates ade-interleaver, 118 indicates a judging device which judges the outputof the second decoder 115 to output an estimated value of the originalinformation bit list. The received signals y₁, y₂, y₃ are signals thatare formed by allowing the information bit list x₁ and the redundant bitlists x₂, x₃ to include influences from noise and phasing in thetransmission path.

In this turbo decoder, first, the first decoder 111 calculates thelogarithm likelihood ratio: L (x_(1k)′) (where k refers to the time) ofestimated information bit: x_(1k)′ from received signals: y_(1k) andy_(2k). In this case, the logarithm likelihood ratio: L (x_(1k)′) isrepresented by the following equation: $\begin{matrix}\begin{matrix}{{L\left( x_{1k}^{\prime} \right)} = \quad {y_{1k} + {{La}\left( x_{1k} \right)} + {{Le}\left( x_{1k} \right)}}} \\{= \quad {{Ln}\quad \frac{\Pr\left( \left. {x_{1k} = {1{\left\{ Y \right\}}}} \right) \right.}{\Pr\left( \left. {x_{1k} = {0{\left\{ Y \right\}}}} \right) \right.}}}\end{matrix} & (1)\end{matrix}$

In equation (1), Le (x_(1k)) represents external information, La(x_(1k)) represents preliminary information that is external informationpreceding by one, P_(r) (x_(1k)=1|{Y}) represents the probability of anactually transmitted information bit: x_(1k) being 1 under the conditionthat the entire list {Y} of the received signals has been received, andP_(r) (x_(1k)=0{y}) represents the probability of an actuallytransmitted information bit: x_(1k) being 0 under the condition that theentire list {Y} of the received signals has been received. In otherwords, equation (1) finds a ratio of the probability of the informationbit: x_(1k) becoming 1 to the probability of the information bit: x_(1k)being 0.

The adder 112 calculates external information to be given to the seconddecoder 115 from a logarithm likelihood ratio that is the result of theabove-mentioned calculation. Based upon the equation (1), the externalinformation: Le (x_(1k)) is represented by the following equation:

Le (x _(1k))=L(x _(1k))−y _(1k) −La (x _(1k))  (2)

Since no preliminary information has been given at the time of the firstdecoding process, La (x_(1k))=0.

In the interleavers 113 and 114, in order to make the received signal:y_(1k) and the external information: Le (x_(1k)) coincident with thetime of the received signal: y₃, the signals are re-arranged. Then, inthe same manner as the first decoder 111, based upon the receivedsignal: y₁ and the received signal: y₃ as well as the externalinformation: Le (x_(1k)) preliminarily calculated, the second decoder115 calculates a logarithm likelihood ratio: L (x_(1k)′). Thereafter, inthe same manner as the adder 112, the adder 116 calculates the externalinformation Le (x_(1k)) by using equation (2). At this time, theexternal information, rearranged by the interleave 117, is fed back tothe first decoder 111 as the preliminary information: La (x_(1k)).

Finally, in the turbo decoder, the above-mentioned processes arerepeatedly executed predetermined times so that it is possible tocalculate a logarithm likelihood ratio with higher precision, and thejudgment device 118 makes a judgment based upon this logarithmlikelihood ratio, thereby estimating the bit list of the originalinformation. More specifically, for example, the logarithm likelihoodratio shows that “L (x_(1k)′)>0”, the estimated information bit: x_(1k)′is judged as 1, while it shows that “L (x_(1k)′)≦0”, the estimatedinformation bit: x_(1k)′ is judged as 0.

In this manner, in the conventional communication method, by using theturbo code as the error-correction code, even in the case when thesignal point-to-point distance becomes closer as the modulation systemis multi-valued, it becomes possible to greatly improve the transmittingproperty in the voice transmission and data transmission, andconsequently to obtain characteristics superior to the knownconvolutional codes.

However, in the conventional communication method, in order to carry outan error correction with high precision, the turbo encoding process iscarried out on all the information lists on the transmitting side, andon the receiving side, all the encoded signals are decoded, and asoft-judgment is then executed thereon. More specifically, for example,in the case of 16 QAM, a judgment is made with respect to all the 4-bitdata (0000 to 1111: 4-bit constellation), and in the case of 256 QAM, ajudgment is made with respect to all the 8-bit data. Therefore,conventionally, the application of the conventional communication methodthat carries out judgments on all the data as described above causes aproblem of an increase in the number of calculations in the encoder anddecoder in response to the multi-valued levels.

Moreover, the demodulation process is executed by carrying out repeatedcalculations with or without an influence from noise, that is,irrespective of the state of the transmission path. Therefore, even inthe case of a good state of the transmission path, the same number ofcalculations and the same amount of delay as in the case of a bad statethereof are required.

The present invention has been devised to solve the above-mentionedproblems, and its object is to provide a method of and device forcommunications for such a device, which is applicable to anycommunication system using the multi-carrier modem system and thesingle-carrier modem system, and makes it possible to achieve areduction in the number of calculations and to provide a goodtransmitting property, even in the case when there is an increase in theconstellation due to multi-valued levels, and also to greatly reduce thenumber of calculations and the calculation processing time in the caseof a good transmission path.

DISCLOSURE OF THE INVENTION

A communication device in accordance with the present invention, whichuses turbo codes as error-correction codes, is provided with: a turboencoder (corresponding to a turbo encoder 1 in an embodiment which willbe described later) which carries out a turbo encoding process on lowertwo bits in transmission data to output an information bit list of thetwo bits, a first redundant bit list generated in a first convolutionalencoder having the information bit list of the two bits as an input anda second redundant bit list generated in a second convolutional encoderto which the respective information bit lists that have been subjectedto interleave processes are switched and input; a first decoder unit(corresponding to a first decoder 11, an adder 12 and interleavers 13,14) which extracts the information bit list of the two bits and thefirst redundant bit list from a received signal, and calculates theprobability information of estimated information bits by using theresults of the extraction and probability information that has beengiven as preliminary information (in some cases, not given); a seconddecoder unit (corresponding to a second decoder 15, an adder 16 and adi-interleaver 17) which extracts the information bit list of the twobits and the second redundant bit list, and again calculates theprobability information of estimated information bits by using theresults of the extraction and the probability information from the firstdecoder unit to inform the first decoder unit of the results as thepreliminary information; a first estimating unit (corresponding to afirst judging device 18 and a second judging device 20) which, basedupon the results of the calculation processes of probability informationby the first and second decoder unit that are repeatedly executed,estimates the information bit list of the original lower two bits foreach of the calculation processes; an error correction unit(corresponding to a first R/S decoder 19 and a second R/S decoder 21)which carries out an error checking process on the estimated informationbit list by using an error correction code, and terminates the repeatingprocess at the time when a judgment shows that the estimation precisionexceeds a predetermined reference, as well as simultaneously carryingout an error-correction process on the estimated information bit list ofthe original lower two bits by using an error correction code; and asecond estimating unit (corresponding to a third judging device 22)which hard-judges the other upper bits in the received signal so as toestimate an information bit list of the original upper bits.

A communication device in accordance with the next invention, whichserves as a receiver using the turbo codes as error-correction codes, isprovided with: a first decoder unit which extracts an information bitlist of the two bits and a first redundant bit list from a receivedsignal, and calculates the probability information of estimatedinformation bits by using the results of the extraction and probabilityinformation that has been given as preliminary information (in somecases, not given); a second decoder unit which extracts the informationbit list of the two bits and a second redundant bit list, and againcalculates the probability information of estimated information bits byusing the results of the extraction and the probability information fromthe first decoder unit to inform the first decoder unit of the resultsas the preliminary information; a first estimating unit which, basedupon the results of the calculation processes of probability informationby the first and second decoder unit that are repeatedly executed,estimates the information bit list of the original lower two bits foreach of the calculation processes; an error correction unit whichcarries out an error checking process on the estimated information bitlist by using an error correction code, and terminates the repeatingprocess at the time when a judgment shows that the estimation precisionexceeds a predetermined reference, as well as simultaneously carryingout an error-correction process on the estimated information bit list ofthe original lower two bits by using an error correction code; and asecond estimating unit which hard-judges the other upper bits in thereceived signal so as to estimate an information bit list of theoriginal upper bits.

In a communication device in accordance with the next invention, theerror correction unit carries out an error checking process each timethe information bit list of the lower two bits is estimated, andterminates the repeating process at the time when a judgment shows that“no error exists” in the estimated information bit list.

In a communication device in accordance with the next invention, theerror correction unit carries out an error checking process each timethe information bit list of the lower two bits is estimated, andterminates the repeating process at the time when a judgment shows that“neither the information bit list estimated based upon the probabilityinformation from the first decoding unit nor the information bit listestimated based upon the probability information from the seconddecoding unit includes any error” in the estimated information bit list.

In a communication device in accordance with the next invention, theerror correction unit carries out the repeating process for apredetermined number of times, and after the bit error rate has beenreduced, an error-correction process is carried out on the estimatedinformation bit list of the original lower two bits by using errorcorrection codes.

A communication device in accordance with the next invention, whichserves as a receiver using the turbo codes as error-correction codes, isprovided with: a turbo encoder which carries out a turbo encodingprocess on lower two bits in transmission data to output an informationbit list of the two bits, a first redundant bit list generated in afirst convolutional encoder having the information bit list of the twobits as an input and a second redundant bit list generated in a secondconvolutional encoder to which the respective information bit lists thathave been subjected to interleave processes are switched and input.

A communication method in accordance with the next invention, which usesturbo codes as error-correction codes, is provided with: a turboencoding step of carrying out a turbo encoding process on lower two bitsin transmission data to output an information bit list of the two bits,a first redundant bit list generated in a first convolutional encoderhaving the information bit list of the two bits as an input and a secondredundant bit list generated in a second convolutional encoder to whichthe respective information bit lists that have been subjected tointerleave processes are switched and input; a first decoding step ofextracting the information bit list of the two bits and the firstredundant bit list from a received signal, as well as calculating theprobability information of estimated information bits by using theresults of the extraction and probability information that has beengiven as preliminary information (in some cases, not given); a seconddecoding step of further extracting the information bit list of the twobits and the second redundant bit list, as well as again calculating theprobability information of estimated information bits by using theresults of the extraction and the probability information from the firstdecoding step to inform the first decoding step of the results as thepreliminary information; a first estimating step of estimating theinformation bit list of the original lower two bits for each of thecalculation processes, based upon the results of the calculationprocesses of probability information by the first and second decodingsteps that are repeatedly executed; an error-correction step of carryingout an error checking process on the estimated information bit list byusing an error correction code, and terminating the repeating process atthe time when a judgment shows that the estimation precision exceeds apredetermined reference, as well as simultaneously carrying out anerror-correction process on the estimated information bit list of theoriginal lower two bits by using an error correction code; and a secondestimating step of hard-judging the other upper bits in the receivedsignal so as to estimate an information bit list of the original upperbits.

In the communication method in accordance with the next invention, theerror correction step is designed to carry out an error checking processeach time the information bit list of the lower two bits is estimated,and also to terminate the repeating process at the time when a judgmentshows that “no error exists” in the estimated information bit list.

In the communication method in accordance with the next invention, theerror correction step is designed to carry out an error checking processeach time the information bit list of the lower two bits is estimated,and also to terminate the repeating process at the time when a judgmentshows that neither the information bit list estimated based upon theprobability information from the first decoding step nor the informationbit list estimated based upon the probability information from thesecond decoding step includes “any error” in the estimated informationbit list.

In the communication method in accordance with the next invention, theerror correction step is designed to carry out the repeating process fora predetermined number of times, and after the bit error rate has beenreduced, an error-correction process is carried out on the estimatedinformation bit list of the original lower two bits by using errorcorrection codes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing that shows constructions of an encoder and a decoderthat are used in a communication device in accordance with the presentinvention;

FIG. 2 is a drawing that shows a construction of a transmitting systemof a transmitter in accordance with the present invention;

FIG. 3 is a drawing that shows a construction of a receiving system inaccordance with the present invention;

FIG. 4 is a drawing that shows layouts of signal points in variousdigital modulations;

FIG. 5 is a drawing that shows a circuit construction of a turbo encoder1;

FIG. 6 is a drawing that shows a construction of a conventional turboencoder; and

FIG. 7 is a drawing that shows a construction of a conventional turboencoder.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of a method of and device for communications in accordancewith the present invention in detail while referring to the accompanyingdrawings. However, the present invention is not intended to be limitedby these embodiments.

FIG. 1 is a drawing that shows constructions of an encoder (a turboencoder) and a decoder (a combination of a turbo decoder, a hard-judgingdevice and a R/S (Reed Solomon code) decoder) used in a communicationdevice in accordance with the present invention. Precisely, FIG. 1(a)shows the construction of the encoder of the present embodiment, whileFIG. 1(b) shows the construction of the decoder thereof. In thecommunication device in accordance with the present embodiment, both theconstructions of the encoder and decoder are installed so that it ispossible to provide a data error-correction capability with highprecision, and consequently to obtain a superior transmitting propertyin data communication and voice communication. For convenience ofexplanation, it is assumed that both the constructions are provided.However, for example, only the encoder may be installed in atransmitter, or only the decoder may be installed in a receiver.

In the encoder in FIG. 1(a), reference number 1 indicates a turboencoder that uses turbo codes as error-correction codes so as to providea performance close to the Shannon limit. For example, with respect toan input of two-bit information bits the turbo encoder 1 outputs two-bitinformation bits and two-bit redundant bits, and the respectiveredundant bits are generated so as to uniform the correctioncapabilities with respect to the information bits on the receiving side.

In the decoder shown in FIG. 1(b), reference numeral 11 indicates afirst decoder which calculates the logarithm likeliness ratio from areceived signal: Lcy (corresponding to received signals: y₂, y₁, y_(a),as will be described later), 12 and 16 indicate adders; 13 and 14indicate interleavers, 15 indicates a second decoder for calculating thelogarithm likeliness ratio from the received signal: Lcy (correspondingto received signals: y₂, y₁, y_(b), as will be described later), 17indicates a de-interleaver, 18 indicates a first judging device whichjudges the output of the first decoder 15 to output an estimated valueof the original information bit list, 19 indicates a first R/S decoderwhich decodes Reed Solomon codes to output an information bit list withhigher precision, 20 indicates a second judging device which judges theoutput of the second decoder 15 to output an estimated value of theoriginal information bit list, 21 indicates a second R/S decoder whichdecodes Reed Solomon codes to output an information bit list with higherprecision, and 22 indicates a third judging device for hard-judging thereceived signal Lcy (corresponding to received signals: y₃, y₄ . . . ,as will be described later) to output an estimated value of the originalinformation bit list.

Prior to explaining the operations of the encoder and decoder, anexplanation will be briefly given of the basic operation of thecommunication device in the present invention by referring to thefigures. For example, with respect to the cable-type digitalcommunication system for carrying out data communication by using theDMT (Discrete Multi Tone) modem system, there are xDSL communicationsystems including an ADSL (Asymmetric Digital Subscriber Line)communication system that executes a high-speed digital communicationwith several mega bits/second by using the existing telephone lines andan HDSL (high-bit-rate Digital Subscriber Line) communication system.Here, these systems are standardized in T1.413 of the ANSI, etc. In theexplanation of the present embodiment, for example, a communicationdevice that is applicable to the ADSL is used.

FIG. 2 is a drawing that shows the construction of a transmitting systemof a communication device in accordance with the present invention. InFIG. 2, in the transmitting system, the transmission data is multiplexedby a multiplex/synch control (corresponding to a MUS/SYNC CONTROL in theFigure) 41, and error-detecting codes are added to the transmission datathat has been multiplexed in cyclic redundancy checks (CRC) 42, 43, andFEC-use codes are added thereto and a scrambling process is also appliedthereto in forward error corrections (corresponding to SCRAM & FEC) 44,45.

There are two paths from the multiplex/synch control 41 to a toneordering 49, and one is an interleaved data buffer path containing theinterleave 46, and the other is a fast data buffer path that does notcontain the interleave; thus, for example, the interleaved data bufferpath for executing an interleaving process has a greater delay.

Thereafter, the transmission data is subjected to a rate convertingprocess in rate converters (corresponding to RATE-CONVERTORS) 47, 48,and then subjected to a tone ordering process in the tone ordering(corresponding to TONE ORDERRING) 49. Based upon the transmission dataafter the tone ordering process, constellation data is formed in aconstellation encoder/gain scaling (corresponding to CONSTELLATION ANDGAIN SCALING) 50, and this is subjected to an inverse Fast Fouriertransform in an inverse Fast Fourier transform section (corresponding toIFFT: Inverse Fast Fourier transform) 51.

Finally, after the Fourier transform, the parallel data is converted toserial data in an input parallel/serial buffer (corresponding to INPUTPARALLEL/SERIAL BUFFER) 52, and the digital waveform is converted to ananalog waveform in analog processing/digital-analog converter(corresponding to ANALOG PROCESSING AND DAC) 53; then, after having beensubjected to a filtering process, the resulting transmission data istransmitted to a telephone line.

FIG. 3 is a drawing that shows a construction of a receiving system ofthe communication device in accordance with the present invention. InFIG. 3, in the receiving system, the received data (corresponding to theabove-mentioned transmission data) is subjected to a filtering processin an analog processing/analog-digital converter (corresponding toANALOG PROCESSING AND ADC in the Figure) 141, and the analog waveform isconverted to a digital waveform; thereafter, this is subjected to anadaptive equalization process with respect to the time domain in a timedomain equalizer (corresponding to TEQ) 142.

With respect to the data having been subjected to the adaptiveequalization process, this is converted from serial data to paralleldata in an input serial/parallel buffer (corresponding to INPUTSERIAL/PARALLEL BUFFER) 143, and this parallel data is subjected to afast Fourier transform in a fast Fourier transform section(corresponding to FET: Fast Fourier transform) 144; thereafter, this issubjected to an adaptive equalization process with respect to thefrequency domain in a frequency domain equalizer (corresponding to FEQ)145.

The data, which has been subjected to the adaptive equalization processwith respect to the frequency domain, is subjected to a decoding process(most likeliness decoding method) and a tone ordering process in aconstellation decoder/gain scaling (corresponding to CONSTELLATIONDECODER AND GAIN SCALING) 146 and a tone ordering (corresponding to TONEORDERING) 147 so that this is converted to serial data. Thereafter, thisis subjected to processes, such as a rate converting process by rateconverters (corresponding to RATE-CONVERTER) 148, 149, a de-interleavingprocess in a de-interleave (corresponding to DEINTERLEAVE) 150, an FECprocess and a de-scrambling process in forward error corrections(corresponding to DESCRAM & FEC) 151, 152, and a cyclic redundancy checkin cyclic redundancy checks (corresponding to cyclic redundancy checks)153, 154; thus, the received data is finally reproduced from amultiplex/synch control (corresponding to MUX/SYNC CONTROL) 155.

In the communication device according to the present invention, the twopaths are provided respectively in the receiving system and transmittingsystem, and by using these two paths properly or using these two pathsat the same time, it is possible to realize a low-transmission delay anddata communication with high rates.

The encoder, shown in FIG. 1(a), is positioned at the constellationencoder/gain scaling 50 in the transmitting system, and the decoder,shown in FIG. 1(b), is positioned at the constellation decoder/gainscaling 146 in the receiving system.

Detailed explanation about the operations of the encoder (transmittingsystem) and the decoder (receiving system) will be given here. First, anexplanation will be given of the operation of the encoder shown in FIG.1(a). In the present embodiment, with respect to the multi-valueQuadrature Amplitude Modulation (QAM), for example, a 16 QAM system isadopted. Moreover, in the encoder of the present embodiment, differentfrom the conventional technique that executes a turbo encoding processon all the input data, the turbo encoding process is executed on theinput data of the lower two bits as illustrated in FIG. 1(a), and withrespect to the other upper bits, the input data, as it is, is output.

The following description will discuss why only the lower two bits ofthe input data are subjected to the turbo encoding process. FIG. 4 is adrawing that shows the layout of signal points in various digitalmodulations; and more specifically, FIG. 4(a) shows the layout of signalpoints in the 4-phase PSK (Phase Shift Keying) system, FIG. 4(b) showsthe layout of signal points in the 16 QAM system, and FIG. 4(c) showsthe layout of signal points in the 64 QAM system.

For example, when, in the layout of signal points in all the modulationsystems, the received signal points are a or b positions, on thereceiving side, normally, the data having the most likelihood isestimated as the information bit list (transmission data) through asoft-judgment. In other words, the signal point having the closestdistance to the received signal point is judged as the transmissiondata. However, at this time, for example, when attention is given to thereceived signal points a and b in FIG. 5, it is found that the fourpoints, which are closest to the received signal point, have lower twobits represented by (0,0) (0, 1) (1, 0) (1, 1), in any of the cases(corresponding to FIGS. 4(a), (b) and (c)). Therefore, in the presentembodiment, with respect to the lower two bits of the four signal points(the four points closest to received signal point) that are more likelyto have degradation in the characteristics, the turbo encoding processhaving a superior error-correction capability is applied thereto, and asoft-judgment is carried out on the receiving side. In contrast, withrespect to the other higher bits that are less likely to havedegradation in the characteristics, these bits are output as they are,and a hard-judgment is made on the receiving side.

Thus, the characteristics that might have degradation due tomulti-valued levels can be improved, and since the turbo encodingprocess is carried out only on the lower two bits of the receivedsignal, it is possible to greatly reduce the number of calculations ascompared with the conventional technique that applies the turbo encodingprocess to all the bits.

The following description will discuss the operation of the turboencoder 1 shown in FIG. 1(a) that carries out the turbo encoding processon the input lower two bits of the received data: u₁ and u₂. FIG. 5 is adrawing that shows the circuit construction of the turbo encoder 1. InFIG. 5, reference number 31 indicates a first recursive systemconvolutional encoder, 32 and 33 indicate interleavers, and 34 indicatesa second recursive system convolutional encoder. In the turbo encoder 1,the transmission data: u₁ and u₂ corresponding the information list,redundant data: u_(a) obtained by encoding the transmission data throughthe process of the first recursive system convolutional encoder 31 andredundant data: u_(b) (having time different from the other data)obtained by encoding the transmission data that has beeninterleave-processed through the second recursive system convolutionalencoder 34, are simultaneously output.

Moreover, normally, in the known turbo encoder, for example, thetransmission data: u₂ is input to the respective adders 60 and 62 on thepreceding stages of the first recursive system convolutional encoder 31and the second recursive system convolutional encoder 34, and the othertransmission data: u₁ is input to the respective adders 61 and 63 on thesucceeding stages of the first recursive system convolutional encoder 31and the second recursive system convolutional encoder 34; thus,redundant data: u_(a) and u_(b) are output as the outputs of therespective encoders. However, in the turbo encoders of this type, thenumber of delay devices is different between the transmission data: u₁and u₂, with the result that a bias occurs in the weights of theredundant bits, and the resulting problem is that estimation precisionsof the transmission data: u₁ and u₂ on the receiving side obtained byusing the redundant data: u_(a) and u_(b) are not uniformed.

Therefore, in order to uniform the estimation precisions of thetransmission data: u₁ and u₂, for example, the transmission data: u₂ isinput to the adder 60 on the preceding stage of the first recursivesystem convolutional encoder 31, and the transmission data: u₂ that hasbeen subjected to the interleave process is input to the adder 63 on thesucceeding stage of the second recursive system convolutional encoder34. Furthermore, the other transmission data: u₁ is input to the adder61 on the succeeding stage of the first recursive system convolutionalencoder 31, and the transmission data: u₁ that has been subjected to theinterleave process is input to the adder 62 on the preceding stage ofthe second recursive system convolutional encoder 34.

In this manner, by the effects of the interleave process, it is possibleto improve the error correction capability with respect to burst-typedata errors, and by switching the inputs of the transmission data: u₁and u₂ between the first recursive system convolutional encoder 31 andthe second recursive system convolutional encoder 34, it becomespossible to uniform the estimation precisions of the transmission data:u₁ and u₂ on the receiving side.

Operation of the decoder shown in FIG. 1(b) will be explained. Forexample, the 16 QAM system is adopted as the multi-value quadratureamplitude modulation (QAM). In the decoder in the present embodiment, aturbo decoding process is carried out on the lower two bits of thereceived data so that the original transmission data is estimated by asoft judgment, and with respect to the other upper bits, the receiveddata is subjected to a hard judgment in the third judging device 22 sothat the original transmission data is estimated. Here, the receivedsignals Lcy: y₄, y₃, y₂, y₁, y_(a), y_(b) are signals obtained byallowing the respective outputs on the transmitting side: u₄, u₃, u₂,u₁, u_(a), u_(b) to contain influences from noise and phasing due to thetransmission path.

First, in the turbo decoder, upon receipt of the signals Lcy: y₄, y₃,y₂, y₁, y_(a), y_(b), the first decoder 11 extracts the received signalsLcy: y₂, y₁, y_(a), and calculates the logarithm likelihood ratio:L(U_(1k)′), L(U_(2k)′) (with k representing the time) of informationbits: u_(1k)′, u_(2k)′ (corresponding to the original transmission data:u_(1k), u_(2k)) estimated by these received signals. Here, with respectto the decoder for calculating the logarithm likelihood ratio, forexample, the known maximum posterior probability decoder (MAP algorithm:Maximum A-Posteriori) is often used; however, for example, the knownVitabi decoder may be used.

In this case, the logarithm likelihood ratio: L (u_(1k)′), L(u_(2k)′)are represented by the following equations: $\begin{matrix}\begin{matrix}{{L\left( u_{1k}^{\prime} \right)} = \quad {L_{cy} + {{La}\left( u_{1k} \right)} + {{Le}\left( u_{1k} \right)}}} \\{= \quad {{Ln}\quad \frac{\Pr\left( \left. {u_{1k} = {1{\left\{ {Lcy} \right\}}}} \right) \right.}{\Pr\left( \left. {u_{1k} = {0{\left\{ {Lcy} \right\}}}} \right) \right.}}}\end{matrix} & (3) \\\begin{matrix}{{L\left( u_{2k}^{\prime} \right)} = \quad {L_{cy} + {{La}\left( u_{2k} \right)} + {{Le}\left( u_{2k} \right)}}} \\{= \quad {{Ln}\quad \frac{\Pr\left( \left. {u_{2k} = {1{\left\{ {Lcy} \right\}}}} \right) \right.}{\Pr\left( \left. {u_{2k} = {0{\left\{ {Lcy} \right\}}}} \right) \right.}}}\end{matrix} & (4)\end{matrix}$

In these equations, Le (u_(1k)), Le (u_(2k)) represent externalinformation; La (u_(1k)), La (u_(2k)) represent pre-information that isexternal information preceding by one; P_(r) (u_(1k)=1|{Lcy}) representsthe posterior probability of the actually transmitted information bit:u_(1k) being 1, under the condition that the received signals Lcy: y₂,y₁, y_(a) have been received; P_(r) (u_(1k)=0|{Lcy}) represents theposterior probability of the actually transmitted information bit:u_(1k) being 0; P_(r) (u_(2k)=1|{Lcy}) represents the posteriorprobability of the actually transmitted information bit: u_(2k) being 1under the condition that the received signals Lcy: y₂, y₁, y_(a) havebeen received; and P_(r) (u_(2k)=0|{Lcy}) represents the posteriorprobability of the actually transmitted information bit: u_(2k) being 0.In other words, in equations (3) and (4), the probability of u_(2k)being 1 with respect to the probability of u_(2k) being 0 and theprobability of u_(1k) being 1 with respect to the probability of u_(1k)being 0 are found.

In the adder 12, external information for the second decoder 15 iscalculated from the logarithm likelihood ratio that is the abovecalculation result. The external information: Le (u_(1k)), Le (u_(2k))is represented as follows based upon the above-mentioned equations (3)and (4):

Le (u _(1k))=L (u _(1k)′)−Lcy−La (u _(1k))  (5)

Le (u _(2k))=L (u _(2k)′)−Lcy−La (u _(2k))  (6)

In the first decoding process, since no pre-information is found, La(u_(1k))=0, and La (u_(2k))=0.

In the interleavers 13 and 14, the signals are re-arranged based uponthe received signal Lcy and the external information Le (u_(1k)), Le(u_(2k)). Then, in the second decoder 15, in the same manner as thefirst decoder 11, based upon the received signal Lcy andpre-information: La (u_(1k)), La (u_(2k)) that has been preliminarilycalculated, the logarithm likelihood ratio: L (u_(1k)′), L (u_(2k)′ ) iscalculated. In this case, P_(r) (u_(1k)=1|{Lcy}) represents theposterior probability of the actually transmitted information bit:u_(1k) being 1, under the condition that the received signals Lcy: y₂,y₁, y_(b) have been received; P_(r) (u_(1k)=0|{Lcy}) represents theposterior probability of the actually transmitted information bit:u_(1k) being 0; P_(r) (u_(2k)=1|{Lcy}) represents the posteriorprobability of the actually transmitted information bit: u_(2k) being 1under the condition that the received signals Lcy: y₂, y₁, y_(b) havebeen received; and P_(r) (u_(2k)=0|{Lcy}) represents the posteriorprobability of the actually transmitted information bit: u_(2k) being 0.

Thereafter, in the adder 16, in the same manner as the adder 12,external information: Le (u_(1k)), Le (u_(2k)) is calculated by usingequations (5) and (6). At this time, the external information,re-arranged by the de-interleave 17, is fed back to the first decoder 11as the pre-information: La (u_(1k)), La (u_(2k)).

Thereafter, the turbo decoder repeats the above-mentioned processes forpredetermined times so that the logarithm likelihood ratio with higherprecision is calculated. Finally, the first judging device 18 and thesecond judging device 20 judge the signals based upon the logarithmlikelihood ratio so as to estimate the original transmission data. Morespecifically, for example, if the logarithm likelihood ratio shows “L(u_(1k)′)>0”; then, the estimated information bit u_(1k)′ is judged as1, and if it shows “L (u_(1k)′)≦0”; then, the estimated information bitu_(1k)′ is judged as 0; in the same manner, if the logarithm likelihoodratio shows “L (u_(2k)′)>0”; then, the estimated information bit u_(2k)′is judged as 1, and if it shows “L (u_(2k)′)≦0”; then, the estimatedinformation bit u_(2k)′ is judged as 0. With respect to the receivedsignals Lcy: y₃, y₄, . . . that are simultaneously received, they arehard-judged by using the third judging device 22.

Finally, in the first R/S decoder 19 and the second R/S decoder 21 carryout error checking processes through a predetermined method using ReedSolomon codes, and the above-mentioned repeating process is terminatedat the time when it is judged that the estimation precision has exceededa predetermined reference. Then, an error-correction process is carriedout by using Reed Solomon codes on the original transmission data thathas been estimated by the respective judging devices in theabove-mentioned manner, thereby making it possible to outputtransmission data having higher estimation precision.

Referring to specific examples, the following description will discussestimating methods of the original transmission data by using the firstR/S decoder 19 and the second R/S decoder 21. Three methods are assumedas the specific examples. In the first method, for example, each timethe first judging device 18 or the second judging device 20 estimatestransmission data, the corresponding first R/S decoder 19 or second R/Sdecoder 21 alternately carries out an error checking process, and at thetime when either one of the R/S decoders has made a judgment that “noerror exists”, the above-mentioned repeating process by the turboencoder is terminated; then, an error-correction process is carried outon the original transmission data that has been estimated as describedabove by using Reed Solomon codes, thereby making it possible to outputtransmission data having higher estimation precision.

In the second method, for example, each time the first judging device 18or the second judging device 20 estimates transmission data, thecorresponding first R/S decoder 19 or second R/S decoder 21 alternatelycarries out an error checking process, and at the time when the two R/Sdecoders have made a judgment that “no error exists”, theabove-mentioned repeating process by the turbo encoder is terminated;then, an error-correction process is carried out on the originaltransmission data that has been estimated as described above by usingReed Solomon codes, thereby making it possible to output transmissiondata having higher estimation precision.

Moreover, in the third method, in an attempt to solve a problem in whichthe judgment, “no error exists”, is erroneously made by the first andsecond methods with the result that the repeating process is terminatedand an erroneous correcting operation is carried out, for example, therepeating process is executed for a predetermined number of times, andafter the bit error rate has been reduced to a predetermined level, anerror-correction process is carried out on the original transmissiondata that has been estimated as described above, by using Reed Solomoncodes, thereby making it possible to output transmission data withhigher estimation precision.

In this manner, even if the constellation increases as the modulationsystem is multi-valued, the turbo decoder for carrying out asoft-judgment and an error-correction process using Reed Solomon codeson the lower two bits of the received signal that are more susceptibleto degradation in the characteristics and the judging device forcarrying out a hard-judgment on the other bits of the received signalare provided. Thus, it is possible to reduce the soft-judgment portionshaving a great number of calculations, and also to achieve a goodtransmitting characteristic.

Moreover, by estimating the transmission data using the first R/Sdecoder 19 and the second R/S decoder 21, it becomes possible to reducethe number of iterations, and also to further reduce the soft-judgmentportions having a great number of calculations and the processing timethereof. In addition, in the transmission path having random errors andburst errors in a mixed manner as described in the present embodiment,by adopting the R-S codes (Reed Solomon) for carrying out errorcorrections on a symbol basis and other known error-correction codes ina combined manner, it is possible to obtain a further superiortransmission characteristic.

As described above, the communication device is made applicable tocommunication using the multi-carrier modem system by installing theturbo encoder 1 and the turbo decoder. Therefore, even if theconstellation increases as the modulation system is multi-valued, it ispossible to reduce the number of calculations and the calculationprocessing time, and also to achieve a good transmitting characteristic.The 16 QAM system is taken as the modulation method, however, thepresent invention is not intended to be limited by this method, and thesame effects may be obtained even in the case when another modulationmethod (such as 256 QAM) is used.

As described above, in accordance with the present invention, atransmitter having a construction including a turbo encoder and areceiver having a construction including decoding unit are installed.Therefore, even if the constellation increases as the modulation systemis multi-valued, it is possible to provide a communication device whichcan reduce the number of calculations and the calculation processingtime, and also to achieve a good transmitting characteristic.

In accordance with the next invention, the first estimating unit forcarrying out a soft-judging process on the lower two bits of a receivedsignal that is more susceptible to degradation in the characteristics,an error-correction unit for carrying out an error-correction process byusing Reed Solomon codes and the second estimating unit for carrying outa hard-judging process on the other bits of the received signal areinstalled. Therefore, it is possible to provide a communication devicewhich, even in the case when the constellation increases as themodulation system is multi-valued, makes it possible to reducecalculations in the soft-judging process having a great number ofcalculations, and also to achieve a good transmitting characteristic.

In accordance with the next invention, at the time when theerror-correction unit makes a judgment that “no error exists” in theinformation bit list that has been preliminarily estimated, therepeating process in the turbo decoding process is terminated.Therefore, it is possible to provide a communication device which canreduce the number of iterations, and further reduce calculations in thesoft-judging process having a great number of calculations as well asthe calculation processing time.

In accordance with the next invention, at the time when theerror-correction unit makes a judgment that “neither the information bitlist estimated based upon the probability information from the firstdecoder unit nor the information bit list estimated based upon theprobability information from the second decoder unit includes any error”the repeating process in the turbo decoding process is terminated.Therefore, it is possible to provide a communication device which canreduce calculations in the soft-judging process having a great number ofcalculations as well as the calculation processing time, and alsoimprove the estimation precision in the information bits.

In accordance with the next invention, the error-correction unit allowsthe repeating process to be carried out as many as a predeterminednumber. Therefore, it is possible to provide a communication devicewhich can reduce the number of iterations, and further reducecalculations in the soft-judging process having a great number ofcalculations as well as the calculation processing time.

In accordance with the next invention, it is possible to improvecharacteristics that are susceptible to degradation due to theapplication of a multi-valued system, and since the turbo encoding iscarried out on only the lower two bits of the received signal, it ispossible to provide a communication device which can greatly reduce thenumber of calculations as compared with the conventional technique inwhich all the bits are turbo encoded. Moreover, by the effects of theinterleave process, it is possible to improve the error correctioncapability with respect to burst-type data errors, and by switching theinputs of information bits between the first convolutional encoder andthe second convolutional encoder, it becomes possible to provide acommunication device which can uniform the estimation precisions of theinformation bits on the receiving side.

In accordance with the next invention, even if the constellationincreases as the modulation system is multi-valued, it is possible toprovide a communication method which can reduce the number ofcalculations and the calculation processing time, and also achieve agood transmitting characteristic.

In accordance with the next invention, at the time when theerror-correction step makes a judgment that “no error exists” in theinformation bit list that has been preliminarily estimated in theerror-correction step, the repeating process in the turbo decodingprocess is terminated. Therefore, it is possible to provide acommunication method which can reduce the number of iterations, andfurther reduce calculations in the soft-judging process having a greatnumber of calculations as well as the calculation processing time.

In accordance with the next invention, at the time when theerror-correction step makes a judgment that “neither the information bitlist estimated based upon the probability information from the firstdecoding step nor the information bit list estimated based upon theprobability information from the second decoding step includes anyerror” the repeating process in the turbo decoding process isterminated. Therefore, it is possible to provide a communication methodwhich can reduce calculations in the soft-judging process having a greatnumber of calculations as well as the calculation processing time, andalso improve the estimation precision in the information bits.

In accordance with the next invention, the error-correction step allowsthe repeating process to be carried out as many as a predeterminednumber. Therefore, it is possible to provide a communication devicewhich can reduce the number of iterations, and further reducecalculations in the soft-judging process having a great number ofcalculations as well as the calculation processing time.

INDUSTRIAL APPLICABILITY

As described above, a communication device and a communication method ofthe present invention are suitable for communications (data, voice,image, etc.) using a method such as the DMT (discrete multi tone) modemsystem and the OFDM (orthogonal frequency division multiplex) modemsystem, which are examples of the multi-carrier modem system, and inparticular, applicable to a communication device which uses turbo codesas error-correction codes.

What is claimed is:
 1. A communication device, which uses turbo codes aserror-correction codes, comprising: a turbo encoder which carries out aturbo encoding process on lower two bits in transmission data to outputan information bit list of the two bits, a first redundant bit listgenerated in a first convolutional encoder having the information bitlist of the two bits as an input and a second redundant bit listgenerated in a second convolutional encoder to which the respectiveinformation bit lists that have been subjected to interleave processesare switched and input; a first decoder unit which extracts theinformation bit list of the two bits and the first redundant bit listfrom a received signal, and calculates the probability information ofestimated information bits by using the results of the extraction andprobability information that has been given as preliminary information;a second decoder unit which extracts the information bit list of the twobits and the second redundant bit list, and again calculates theprobability information of estimated information bits by using theresults of the extraction and the probability information from the firstdecoder unit to inform the first decoder unit of the results as thepreliminary information; a first estimating unit which, based upon theresults of the calculation processes of probability information by thefirst and second decoder units that are repeatedly executed, estimatesthe information bit list of the original lower two bits for each of thecalculation processes; an error correction unit which carries out anerror checking process on the estimated information bit list by using anerror correction code, and terminates the repeating process at the timewhen a judgment shows that the estimation precision exceeds apredetermined reference, as well as simultaneously carrying out anerror-correction process on the estimated information bit list of theoriginal lower two bits by using an error correction code; and a secondestimating unit which hard-judges the other upper bits in the receivedsignal so as to estimate an information bit list of the original upperbits.
 2. The communication device according to claim 1, wherein theerror correction unit carries out an error checking process each timethe information bit list of the lower two bits is estimated, andterminates the repeating process at the time when a judgment shows that“no error exists” in the estimated information bit list.
 3. Thecommunication device according to claim 1, wherein the error correctionunit carries out an error checking process each time the information bitlist of the lower two bits is estimated, and terminates the repeatingprocess at the time when a judgment shows that “neither the informationbit list estimated based upon the probability information from the firstdecoding unit nor the information bit list estimated based upon theprobability information from the second decoding unit includes anyerror” in the estimated information bit list.
 4. The communicationdevice according to claim 1, wherein the error correction unit carriesout the repeating process for a predetermined number of times, and afterthe bit error rate has been reduced, an error-correction process iscarried out on the estimated information bit list of the original lowertwo bits by using error correction codes.
 5. A communication device,which serves as a receiver using turbo codes as error-correction codes,comprising: a first decoder unit which extracts an information bit listof the two bits and a first redundant bit list from a received signal,and calculates the probability information of estimated information bitsby using the results of the extraction and probability information thathas been given as preliminary information (in some cases, not given); asecond decoder unit which extracts the information bit list of the twobits and a second redundant bit list, and again calculates theprobability information of estimated information bits by using theresults of the extraction and the probability information from the firstdecoder unit to inform the first decoder unit of the results as thepreliminary information; a first estimating unit which, based upon theresults of the calculation processes of probability information by thefirst and second decoder unit that are repeatedly executed, estimatesthe information bit list of the original lower two bits for each of thecalculation processes; an error correction unit which carries out anerror checking process on the estimated information bit list by using anerror correction code, and terminates the repeating process at the timewhen a judgment shows that the estimation precision exceeds apredetermined reference, as well as simultaneously carrying out anerror-correction process on the estimated information bit list of theoriginal lower two bits by using an error correction code; and a secondestimating unit which hard-judges the other upper bits in the receivedsignal so as to estimate an information bit list of the original upperbits.
 6. The communication device according to claim 5, wherein theerror correction unit carries out an error checking process each timethe information bit list of the lower two bits is estimated, andterminates the repeating process at the time when a judgment shows that“no error exists” in the estimated information bit list.
 7. Thecommunication device according to claim 5, wherein the error correctionunit carries out an error checking process each time the information bitlist of the lower two bits is estimated, and terminates the repeatingprocess at the time when a judgment shows that “neither the informationbit list estimated based upon the probability information from the firstdecoding unit nor the information bit list estimated based upon theprobability information from the second decoding unit includes anyerror” in the estimated information bit list.
 8. The communicationdevice according to claim 5, wherein the error correction unit carriesout the repeating process for a predetermined number of times, and afterthe bit error rate has been reduced, an error-correction process iscarried out on the estimated information bit list of the original lowertwo bits by using error correction codes.
 9. A communication device,which serves as a receiver using the turbo codes as error-correctioncodes, comprising: a turbo encoder which carries out a turbo encodingprocess on lower two bits in transmission data to output an informationbit list of the two bits, a first redundant bit list generated in afirst convolutional encoder having the information bit list of the twobits as an input and a second redundant bit list generated in a secondconvolutional encoder to which the respective information bit lists thathave been subjected to interleave processes are switched and input. 10.A communication method, which uses turbo codes as error-correctioncodes, comprising: a turbo encoding step of carrying out a turboencoding process on lower two bits in transmission data to output aninformation bit list of the two bits, a first redundant bit listgenerated in a first convolutional encoder having the information bitlist of the two bits as an input and a second redundant bit listgenerated in a second convolutional encoder to which the respectiveinformation bit lists that have been subjected to interleave processesare switched and input; a first decoding step of extracting theinformation bit list of the two bits and the first redundant bit listfrom a received signal, as well as calculating the probabilityinformation of estimated information bits by using the results of theextraction and probability information that has been given aspreliminary information (in some cases, not given); a second decodingstep of further extracting the information bit list of the two bits andthe second redundant bit list, as well as again calculating theprobability information of estimated information bits by using theresults of the extraction and the probability information from the firstdecoding step to inform the first decoding step of the results as thepreliminary information; a first estimating step of estimating theinformation bit list of the original lower two bits for each of thecalculation processes, based upon the results of the calculationprocesses of probability information by the first and second decodingsteps that are repeatedly executed; an error-correction step of carryingout an error checking process on the estimated information bit list byusing an error correction code, and terminating the repeating process atthe time when a judgment shows that the estimation precision exceeds apredetermined reference, as well as simultaneously carrying out anerror-correction process on the estimated information bit list of theoriginal lower two bits by using an error correction code; and a secondestimating step of hard-judging the other upper bits in the receivedsignal so as to estimate an information bit list of the original upperbits.
 11. The communication method according to claim 10, herein theerror correction step carries out an error checking process each timethe information bit list of the lower two bits is estimated, and alsoterminates the repeating process at the time when a judgment shows that“no error exists” in the estimated information bit list.
 12. Thecommunication method according to claim 10, wherein the error correctionstep carries out an error checking process each time the information bitlist of the lower two bits is estimated, and terminates the repeatingprocess at the time when a judgment shows that “neither the informationbit list estimated based upon the probability information from the firstdecoding step nor the information bit list estimated based upon theprobability information from the second decoding step includes anyerror” in the estimated information bit list.
 13. The communicationmethod according to claim 10, wherein the error correction step carriesout the repeating process for a predetermined number of times, and afterthe bit error rate has been reduced, an error-correction process iscarried out on the estimated information bit list of the original lowertwo bits by using error correction codes.